This invention relates to the compensation of dimensional changes that occur as a result of temperature changes and, more particularly, to a mechanical structure that provides such temperature compensation.
Nearly all materials have a nonzero coefficient of thermal expansion. Specifically, the dimensions of most materials increase as the temperature increases. This thermal expansion is sometimes put to use in practical devices, but in other cases it can adversely affect the performance of a structure.
In one example, the focusing lenses of an optical system are supported in an optical support structure. The lenses have characteristic focal lengths, and the positions of the lenses and other optical elements are selected to achieve precise focusing. These positions are established at a specific temperature. When the temperature changes, the lengths of the members of the optical support structure also change, with the results that the optical elements are moved relative to each other and the precise optical arrangement is lost.
Various support structures are known in which the mechanical elements interact with each other to achieve a desired and controllable thermal expansion property of the structure that is different from the expansion properties of the individual mechanical elements. In many instances, the mechanical elements are selected to achieve a net zero coefficient of thermal expansion of the structure, so that the positions of the supported elements are unaffected by changes in temperature. In other cases, the structure must exhibit a controlled non-zero expansion as the temperature changes.
An arrangement of telescoping rings having different coefficients of thermal expansion may be used, but such a device requires a number of parts and may be mechanically unwieldy. The structure is also not easily tuned for specific performance. Bimetallic thermal-expansion structures are also known, but they exhibit too much flexure for many applications.
There is accordingly a need for an improved approach to structures that achieve a controlled thermal expansion. The present invention fulfills this need, and further provides related advantages.
The present invention provides a controlled-expansion structure that may be used to achieve zero or nonzero expansion responsive to temperature changes. (As used herein, xe2x80x9cexpansionxe2x80x9d includes an increase in length, no change in length, and a decrease in length.) The controlled-expansion structure has only two parts in one embodiment, which may be made of common materials and readily fabricated. Its performance is determined to a large extent by the selection of mechanical parameters, rather than relying solely on the often-variable thermal expansion properties of the materials. The mechanical structural parameters may be established with precision metalworking processes. The controlled portion of the controlled-expansion structure is very stiff in compression, so that it may be used to fix the dimensions of sensitive mechanical structures.
In accordance with the invention, a controlled-expansion structure having a controlled thermal expansion along a control axis extending between two marker locations over a temperature range comprises a first expansion element having a first marker location and a first contact surface, and a second expansion element having a second marker location and a second contact surface. The first expansion element has a first coefficient of thermal expansion, and the second expansion element has a second coefficient of thermal expansion different from the first coefficient of thermal expansion. A freely movable interface is formed by a contact, typically a sliding contact, between the first contact surface and the second contact surface. The interface is oriented at an interface angle relative to the control axis of greater than 0 but less than (+/xe2x88x92) 90 degrees. The controlled thermal expansion between the two marker locations may be substantially zero, or it may be a positive or negative nonzero value.
The controlled-expansion structure may be implemented in any operable geometry. In one case each of the expansion elements is a hollow cylinder. In another case, each of the first expansion element and the second expansion element comprises an expansion arm and a contact strip affixed to the expansion arm and having the respective contact surface thereon. The present approach may be extended beyond two expansion elements, to three or more expansion elements interrelated to each other in the same manner as the first and second expansion elements.
The first contact surface and the second contact surface may each be planar. The first contact surface and the second contact surface may instead each be nonplanar. In one nonplanar arrangement, one of the contact surfaces serves as a cam surface, and the other serves as a cam follower that rides on the cam surface.
In another embodiment, a controlled-expansion structure having a controlled thermal expansion along a control axis extending between two marker locations over a temperature range comprises a first expansion element having a first marker location, a first contact surface, a first mean perpendicular dimension D1 measured perpendicular to the control axis, a mean height dimension H1 measured between the first marker location and the first contact surface, a first axial coefficient of thermal expansion xcex11H measured parallel to the control axis, and a first transverse coefficient of thermal expansion xcex11D measured parallel to D1. A second expansion element has a second marker location, a second contact surface, a second mean perpendicular dimension D2 measured perpendicular to the control axis and parallel to the first perpendicular dimension, a mean height dimension H2 measured between the second marker location and the second contact surface, a second axial coefficient of thermal expansion xcex12H measured parallel to the control axis, and a second transverse coefficient of thermal expansion xcex12D measured parallel to D1. There is a structural inequality of the first expansion element and the second expansion element such as xcex11H being different from xcex12H, xcex11D being different from xcex12D, and/or D1 being different from D2. Combinations of these inequalities are operable as well. A freely movable interface is formed by a contact between the first contact surface and the second contact surface. The interface is oriented at an interface angle xcex8 relative to the control axis of greater than 0 but less than 90 degrees. This embodiment may utilize any of the compatible features and forms discussed earlier.
In one aspect of this embodiment, xcex11H and xcex11D are the same and are equal to xcex11, and xcex12H and xcex12D are the same and are equal to xcex12. The thermal behavior of this embodiment may be described by
HT=(1+xcex11xcex94T)H1+(1+xcex12xcex94T)H2+xcex94T(xcex11D1xe2x88x92xcex12D2)tan(90xe2x88x92xcex8)
where HT=H1+H2 at an initial temperature and xcex94T is a temperature change from the initial temperature.
The present invention extends to a method for establishing a controlled thermal expansion along a control axis between two marker locations over a temperature range. The method comprises the steps of selecting the magnitude of the controlled thermal expansion, providing a controlled-expansion structure having one of the structures described earlier, and establishing material properties and dimensions of the first expansion element and the second expansion element, and the interface angle to achieve the controlled thermal expansion along the control axis between the first marker location and the second marker location.
The present approach achieves a controllable zero, or positive or negative nonzero coefficient of thermal expansion between the marker locations. The expansion may be made linear by using an angled but flat contact surface. The expansion may be made nonlinear by using an angled and curved contact surface. The controlled-expansion structure may be readily designed for specific desired performance and in this sense xe2x80x9ctunedxe2x80x9d to the expansion requirements.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.